1. Field of Invention
The present invention relates to a flash memory structure and its method of manufacture. More particularly, the present invention relates to a flash memory structure having a common source terminal made from a buried metal layer, whose method of manufacture is also suitable for producing a shallow trench isolation (STI) structure.
2. Description of Related Art
Conventional flash memory is a type of erasable programmable read-only memory (EPROM). There have been many articles written about flash memories. In general, the gate of a flash memory includes a polysilicon floating gate, which is used for storing electric charges, and a control gate, which is used for controlling data access. Therefore, EPROM normally has two gate terminals with the floating gate located below the control gate. The control gate and the word line are usually connected, and the floating gate is usually in a "floating" state. In other words, the floating gate is not in contact with any other circuits. An outstanding property of flash memory is its ability to perform a fast, block-by-block memory erase instead of the slow, bit-by-bit memory erase as in conventional EPROMs. Consequently, operation speed of a flash memory is very fast. Often, the entire memory can be erased within one or two seconds.
FIG. 1 is the top view of a conventional flash memory structure. In FIG. 1, the control gate 10 is used as a word line. The metallic bit line 12 and the control gate 10 run across each other perpendicularly. On each side of the control gate 10, a drain region 14 and a common source 16 are present. There is a contact window 18 above the drain region 14 for coupling electrically with the bit line 12. Furthermore, field oxide layers 13 surround the aforementioned device for insulation.
FIG. 2A is a cross-sectional view taken along line 2I--2I of FIG. 1 that shows a conventional flash memory structure. FIG. 2B is a cross-sectional view taken along line 2II--2II of FIG. 1. First, as shown in FIG. 2A, a common source region 16 is formed within a semiconductor substrate 11. Next, as shown in FIG. 2B, the common source region 16 is isolated by field oxide layers 13, and then control gates 10 are formed above the field oxide layers 13. The control gates 10 can be made from, for example, polysilicon. In order to avoid leakage current or other short-circuit conditions, a minimum distance "a" must be allowed between the control gate 10 and the common source region 16 as shown in FIG. 2B.
In general, this type of flash memory structure has several defects. Firstly, the field oxide insulation structure will produce a rounded corner structure 19 in the common source region 16 close to the control gate 10 when viewed from above (as shown in FIG. 1). Secondly, the field oxide layer in this region has a lateral extension known commonly as the bird's beak as shown in FIG. 2B. Therefore, extra space "a" (as shown in FIG. 1) between the control gate 10 and the common source region 16 must be set aside to prevent unwanted leakage current and short-circuiting.
In general, most integrated circuits must have some form of insulation for isolating one device from its close neighbors. Field oxide layers used to be one of the most commonly used isolating structure. However, the field oxide layer is gradually being replaced by shallow trench isolation (STI) structures. At present, most flash memory structure uses shallow trench isolation. This is because STI has better structural properties than conventional field oxide structure, and furthermore can save chip area. Normally, shallow trench isolation is formed by first performing an anisotropic dry etching operation to form a trench in a substrate, and then depositing some oxide material into the trench.
However, when shallow trench isolation is applied to the fabrication of flash memory structure, area occupied by each device is still large. Moreover, if the common source region and the gate structure are too close together, and the gate oxide layer is too thin, problems such as leakage current or unwanted short-circuiting still have to be dealt with.
In light of the foregoing, there is a need to improve the flash memory structure and method of manufacture.